The present invention relates to the operation of cathode ray tubes (CRT'S) and, more particularly, to mitigating the degrading effects of an external magnetic field upon the operation of a CRT. For purposes of this application the degrading effects discussed concern the convergence, purity and raster rotation of the CRT.
Existing CRT displays suffer intolerable display degradation in the presence of even moderate external magnetic fields. Magnetic shields, i.e. the high permeability housing, surrounding the CRT typically provides adequate shielding against external magnetic fields oriented orthogonal to the CRT viewing axis. However, they provide very little shielding against external fields oriented parallel to the CRT viewing axis. For monochrome displays, external magnetic fields aligned parallel to the CRT viewing axis rotate the raster about the center (zero deflection point) of the raster, causing the corners of the rasters to be obscured by the bezel and causing misregistration with the display and any bezel reference points (e.g. map overlays, variable function key switches with CRT displayed switch labels, etc.). Color shadow mask (CSM) displays are even more susceptible to external fields. In addition to raster rotation, CSM CRT displays lose color purity, convergence, and suffer increased brightness variation due to the external magnetic field induced beam landing errors.
It is known that the harmful effects of an external electromagnetic field may be reduced by providing a bucking field about the CRT. However, existing systems fail to adequately segregate the remedial bucking field from both the CRT display and the external field sensor. Attempts to measure the external field and regulate the bucking field have therefore been of limited success and have resulted in interfering with the normal operation of the display. In contemporary devices the remedial bucking field may operate to distort the CRT display, and/or may be detected and interpreted so as to be indistinguishable from the external field sought to be remedied. Contemporary systems are further limited in that the magnetic susceptibility of the CRT housing and sensor must be taken into account in order to accurately determine the strength and direction of the external field and generate an appropriate remedial field. Variations in the construction of the CRT and/or the sensor may substantially impact assessments of the strength of the external field and the appropriate bucking field.
The present invention provides an apparatus and technique wherein the external field to be remedied is detected at a location spaced from the bucking field to facilitate segregation and accurate assessment of the external field. Moreover, the measurement of the external field is effected within the CRT shielding housing so that the measured field is limited to that which is not adequately shielded by the housing. Consequently, the remedied bucking field will be limited to that necessary to mitigate the effects of the external magnetic field within the housing. The present invention therefore provides an apparatus and technique which overcome limitations of similar contemporary systems and readily accommodates variations in the sensor and CRT construction.